3. Install 3d2png

3d2png is the thumbnail renderer for 3D files. It will render png thumbnails exactly like this extension will display the objects, using the same JS libraries running in Node.js instead of the browser.

To install, clone and install the 3d2png repository:

git clone https://gerrit.wikimedia.org/r/3d2png
cd 3d2png
npm install

On Linux, you'll also need to install a virtual framebuffer in order for 3d2png to be able to headlessly capture the 3D object.

apt-get install xvfb

After having successfully installed 3d2png, we'll need to tell Extension:3D how to call this thumbnail generator service.
Add this to your LocalSettings.php, and make sure to update the paths to match your configuration:

4. Install Extension:MultimediaViewer

MultimediaViewer is not a hard dependency of the 3D extension, but you will need to install it if you want to interact (move, pan, zoom) with the 3D models.
After installing MultimediaViewer, you will need to associate the STL file type with the correct viewer extension, by adding this to your LocalSettings.php file:

$wgMediaViewerExtensions['stl']='mmv.3d';

Configuration

Patent agreement

Special:Upload

To add patent agreement selection options to Special:Upload, edit your wiki's MediaWiki:3d-patents page.
Every line on that page will be turned into an <option>, where everything after the last | character is the text, and everything before is the template that will be added to the upload content.
Check out commons:MediaWiki:3d-patents for an example.

Make sure to create the template(s) that will be added to the content. I.e. Template:3dpatent

Special:UploadWizard

Special:UploadWizard also supports patent agreement, though slightly less flexible.
When uploading a third party STL file, a dialog will be displayed to confirm your upload does not infringe any patents.
When uploading one of your own, it'll also ask to confirm the 3d objects depicted are your own work.
The template added to the upload content will be Template:3dpatent or Template:3dpatent respectively.

To change the phrasing of these messages, these pages can be altered: MediaWiki:Mwe-upwiz-patent-dialog-text-warranty & MediaWiki:Mwe-upwiz-patent-dialog-text-license.

To change which file types require patent agreement, which template is added to the upload content or where the urls point to, change LocalSettings.php. Below are the default settings:

Extension:CommonsMetadata

Lastly, Extension:CommonsMetadata provides a way to track 3D uploads without said patent license, though it is not configurable at all.
It will just check for uploads with a application/sla mime type (currently the only ones supported by Extension:3D) that lack the 3dpatent template. Matching files lacking this template in their content will automatically be added to to Category:Files_with_no_machine-readable_patent.

If you use any other template than 3dpatent, this will not be useful for you, as this is not currently configurable.